The invention relates to a method for adapting a raw NOx concentration value of an internal combustion engine operating with an excess of air.
To further reduce the fuel consumption of motor vehicles with spark-ignition engines, internal combustion engines which are operated with a lean mixture, at least in selected operating ranges, are increasingly being used.
To meet the exhaust emission limit values required, a special exhaust treatment is necessary in the case of such internal combustion engines. NOx storage reduction catalysts, referred to hereafter as NOx storage catalysts for the sake of simplicity, are used for this purpose. On account of their coating, these NOx storage catalysts are capable during a storing phase, also referred to as a loading phase, of adsorbing from the exhaust gas NOx compounds which are produced in lean combustion. During a regeneration phase, the adsorbed or stored NOx compounds are converted into harmless compounds by adding a reducing agent. CO, H2 and HC (hydrocarbons) may be used as the reducing agent for lean-operated spark-ignition internal combustion engines. These are generated by briefly operating the internal combustion engine with a rich mixture and are made available to the NOx storage catalyst as components of the exhaust gas, whereby the stored NOx compounds in the catalyst are broken down.
The adsorption efficiency of such an NOx storage catalyst decreases as the degree of NOx loading increases. The degree of loading is the term used for the quotient of the absolute NOx loading at a given instant and the maximum NOx storage capacity. The calculated degree of loading can be used for controlling the lean-mix and rich-mix cycles of the internal combustion engine. It is evident that, to ascertain the degree of loading, it is necessary to know as accurately as possible both the loading at a given instant and the maximum storage capacity.
The maximum storage capacity can be ascertained on an engine test bench by measuring the NOx stored per unit of time until a state of saturation is reached, while it is not possible for the NOx storage catalyst to become saturated in a motor vehicle for emission reasons. However, this storage capability is subject to an aging process, so that it is necessary to adapt it over the mileage covered by the vehicle. For this purpose, either the value for the loading at a given instant and/or a very accurate value for the raw NOx emission of the internal combustion engine is required. Raw emmissions are generally taken as meaning the emission without exhaust treatment.
One possibility for ascertaining the raw NOx concentration values is to measure a reference internal combustion engine on a test bench and to store the data in suitable characteristic maps. However, reading out from these characteristic maps only produces meaningful results if the raw NOx concentration values of different internal combustion engines of a series do not vary too much. If the variations in the raw NOx concentration values exceed a certain degree, adaptation of the raw NOx concentration values of the internal combustion engine in the motor vehicle is necessary.
It is accordingly an object of the invention to provide a method for adapting a raw NOx concentration value of an internal combustion engine which overcomes the above-mentioned disadvantages of the heretofore-known methods of this general type and which allows to perform the adapting of the raw NOx concentration values in a simple way.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for adapting a raw Nox concentration value of an internal combustion engine operating at least in given operating ranges with an excess of air, wherein the method includes the steps of:
providing an NOx storage reduction catalyst in an exhaust-gas duct of an internal combustion engine;
adsorbing, with the NOx storage reduction catalyst, NOx during a storage phase, when the internal combustion engine is operated with a lean air-fuel mixture;
catalytically converting, with the NOx storage reduction catalyst, NOx stored during the storage phase in a regeneration phase by adding a regenerating agent;
providing an NOx sensor downstream of the NOx storage reduction catalyst;
storing a raw NOx concentration value based on operating parameters of the internal combustion engine in a characteristic map of a memory device of a control device controlling the internal combustion engine;
reading out the raw NOx concentration value from the characteristic map while operating the internal combustion engine; and
adapting, during a cycle including the storage phase and the regeneration phase, the raw NOx concentration value read out from the characteristic map based on an output signal of the NOx sensor.
In other words, the object of the invention is achieved by a method for adapting a raw NOx concentration value of an internal combustion engine operating at least in certain operating ranges with an excess of air, in which:
provided in an exhaust-gas duct of the internal combustion engine is an NOx storage reduction catalyst, which adsorbs NOx during a storage phase, when the internal combustion engine is operated with a lean air-fuel mixture, which catalytically converts the stored NOx in a regeneration phase, with regenerating agent being added,
provided downstream of the NOx storage reduction catalyst is an NOx sensor,
the raw NOx concentration value is stored on the basis of operating parameters of the internal combustion engine in a characteristic map of a memory device of a control device controlling the internal combustion engine, wherein the raw NOx concentration value read out from the characteristic map during operation of the internal combustion engine is adapted during a cycle, formed of the storage phase and the regeneration phase, on the basis of the output signal of the NOx sensor.
The method according to the invention provides that the operating-point-dependent values for the raw NOx concentration of the internal combustion engine are read out from a characteristic map and the adaptation of the variations in concentration takes place on the basis of the output signal of an NOx sensor provided downstream of the NOx storage catalyst, either by modification of a reduction factor, which serves for the calculation of the corrected raw NOx concentration from the raw NOx concentration values, or by direct correction of the values read out from the characteristic map for the raw NOx concentration with a raw concentration correction factor.
Another mode of the invention includes the step of adapting the raw NOx concentration value by changing a reduction factor applied to the raw NOx concentration value, the reduction factor taking into account a steady-state conversion concentration converted by the NOx storage reduction catalyst during a lean operation of the internal combustion engine.
A further mode of the invention includes the step of determining a leakage amount in a lean phase by measuring an NOx concentration downstream of the NOx storage reduction catalyst with the NOx sensor and by integrating the NOx concentration over a duration of the lean phase, calculating a storage amount in the lean phase during a rich phase following the lean phase, calculating an integral of a corrected NOx concentration over the lean phase based on the raw NOx concentration value and the reduction factor, forming a ratio ((DB+SM)/IKK) of a sum of the leakage amount and the storage amount to the integral of the corrected NOx concentration over the lean phase, selectively changing a correction factor for the reduction factor and keeping the correction factor for the reduction factor unchanged dependent on a value of the ratio, and multiplying the reduction factor by the correction factor.
Yet a further mode of the invention includes the step of calculating a corrected raw NOx concentration from an adapted reduction factor by calculating a product of a difference between 1 and the adapted reduction factor and the raw NOx concentration value read out from the characteristic map.
A further mode of the invention includes the step of multiplying the raw NOx concentration value read out from the characteristic map directly with a correction factor for obtaining a pre-corrected value for a raw NOx concentration, and adapting the raw NOx concentration value by changing the correction factor.
Another mode of the invention includes the step of sensing a leakage amount in a lean phase by measuring, with the NOx sensor, an NOx concentration downstream of the NOx storage reduction catalyst and by integrating the NOx concentration over a duration of the lean phase, calculating, during a rich phase following the lean phase, a storage amount in the lean phase, calculating an integral of a corrected NOx concentration over the lean phase based on the raw NOx concentration value and a reduction factor, forming a ratio of a sum of the leakage amount and the storage amount to the integral of the corrected NOx concentration over the lean phase, and selectively changing the correction factor for the raw NOx concentration value and keeping the correction factor for the raw NOx concentration value unchanged dependent on a value of the ratio.
A further mode of the invention includes the step of calculating a corrected raw NOx concentration from the pre-corrected value for the raw NOx concentration by calculating a product of a difference between 1 and the reduction factor and the pre-corrected value for the raw NOx concentration.
Another mode of the invention includes the step of storing the reduction factor in a further characteristic map based on a temperature of the NOx storage reduction catalyst.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for adapting a raw NOx concentration value of an internal combustion engine operating with an excess of air, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.